The Telecommunications Standards Section of the International Telecommunication Union (designated as ITU-T) develops recommendations to facilitate the interoperation of telecommunication networks. Two of these recommendations are designated G.992.1 and G.992.2, which define asymmetric digital subscriber line (ADSL) transceiver technologies.
The G.992.1 and G.992.2 standards both use a multicarrier modulation scheme referred to as discrete multi-tone (DMT) modulation technology. DMT modulation employs multiple carriers or “tones” for both upstream and downstream communication. For example, downstream communication per G.992.1 uses carriers up to index 255, while downstream communication per G.992.2 uses carriers only up to index 127. By working with a large number of carriers rather than a single carrier, the available channel capacity can be maximized thereby optimizing performance of the transmission bandwidth.
Both G.992.1 and G.992.2 have an Annex C that defines a special recommendation for ADSL transceivers under the time compression multiplexing—integrated services digital network (TCM-ISDN) noise environment. Reference to “Annex C” may be to either or both of G.992.1 and G.992.2. TCM-ISDN is defined in ITU-T recommendation G.961 Appendix III. Recommendation G.961 Appendix III describes a ping-pong scheme where the transmission (and reception) of data between two transceivers is performed under the control of a 400 Hz clock referred to as the TCM-ISDN timing reference (TTR). G.992.1 and G.992.2 Annex C central office side transceivers transmit a TTR indication signal during transceiver training so that the customer premise side transceiver can detect and lock its receiver and transmitter to the TTR clock. This TTR indication signal is tone 48 with phase reversal per current versions of G.992.1 and G.992.2. The central office transceiver (ATU-C) transmits the data stream basically in the first half of the TTR period, and the customer transceiver (ATU-R) transmits basically in the second half of the TTR period. Such a ping pong scheme is particularly useful in the context of a communication channel that has high crosstalk interference levels due to the likes of poor quality insulators (e.g., pulp based insulation) within a cable bundle where simultaneous transmission by both the central office and customer transceivers is difficult.
Hyperframe Alignment Based on TTR Detection
The TTR is used to lock the ATU-C local clock frequency, which controls the ATU-C A/D and D/A sampling rates, and the transmitter and receiver symbol rates. The ATU-C transmitter checks the phase of the system TTR, and locks its hyperframe window to the TTR. At the ATU-R, the receiver tracks the received signal from ATU-C transmitter, and locks the local clock to the ATU-C clock frequency. The ATU-R also detects the hyperframe pattern from the received TTR indication signal from the ATU-C, and aligns its symbol counter to the hyperframe pattern (generally referred to as hyperframe alignment). The symbol counter is used to track the symbol index, and is incremented by 1 for each symbol. The counter is reset to zero when it reaches 345 (note that there are 345 symbols in a hyperframe). Thus, the transmitter and receiver are synchronized to the hyperframe. This alignment process is performed during transceiver training of the communication link between two ADSL modems.
Annex C defines a dual bitmap mode (DBM) encoding technique for providing dual bitmaps that are switched synchronized with the hyperframe pattern that is synchronized to the TTR to provide a data stream having dual bit rates. The technique is based on the observation that for short to medium local loops (e.g., less than about three kilometers), the channel signal-to-noise ratio (SNR) can be sufficiently high during NEXT interference to transmit data at a low bit rate. Thus, under certain conditions DBM allows full-duplex operation of TCM-IDSN transceivers by employing different bit rates under NEXT and FEXT interference, respectively. In this sense, the communication channel operating under DBM in a TCM-ISDN environment is effectively two communication channels: one is a FEXT channel and the other is a NEXT channel. A single bitmap mode (SBM, more specifically referred to as FEXT bitmap mode or FBM) encoding technique is also provided. Here, the central office and the remote transceivers transmit data only in FEXT time, and do not transmit data simultaneously (half-duplex mode).
In DBM encoding, bit rates can be changed by changing the bitmaps used to encode the symbols to be transmitted. As is understood by those skilled in the art, a “bitmap” determines the number of bits that can be encoded into each subchannel in a symbol. A “symbol” is the basic unit of information transmitted by the transceiver. The number of bits encoded into each subchannel in a symbol is limited by the quality of the communication channel. The quality of the communication channel can be represented by its SNR. Thus, a system employing DBM includes two bitmaps for providing different data rates: one bitmap for NEXT time and one bitmap for FEXT time. A system employing FBM uses only one bitmap (a FEXT bitmap) since no data signal is transmitted in NEXT time. Because each bitmap used in DBM is associated with different bit rates, the transition from one bit map to the other must be detected to ensure a compatible transmitter-receiver pair. As the standardized frame rate is not a multiple of 400 Hz TTR signal, identifying the boundaries between the NEXT and FEXT channels is not trivial.
According to G.992.1 or G.992.2 Annex C, the ATU-C uses phase changes of a single tone (tone 48) to indicate the transition between the NEXT bitmap and the FEXT bitmap. In particular, the phase of this single tone is toggled by 90 degrees at the transmitter to indicate the transition. Thus, the ATU-R receiver has to detect the phase changes, and then recognize the 345-symbol pattern of FIG. 1a in order to align to the transmitter hyperframe. On long loops and in presence of strong noise, however, it becomes very difficult to detect the phase change of tone 48, and the TTR detection becomes the bottleneck for longer reach. Furthermore, because the tone 48 TTR indication signal indicates only the transition from each bitmap, the receiver needs to search for the pattern implied in FIG. 1a to identify the hyperframe boundary and perform the alignment. This complicates the hyperframe alignment at the receiver and increases the possibility of its failure on long loops.
What is needed, therefore, is a TTR indication signal that is detectable on longer and noisier loops. In a more general sense, there is a need for synchronization of signals or procedures to the TTR.